1. Field of the Invention
This invention relates generally to the fabrication of carbon fiber-carbon matrix composites using forced flow thermal gradient chemical vapor infiltration techniques.
2. Prior Art
Carbon/carbon composites are unique materials in that they exhibit high specific strength, stiffness and toughness, and more importantly, the ability to retain these properties at elevated temperatures (1000-2000.degree. C.). The applications of these materials include rocket nose cones, aircraft disc brakes, heat shields for re-entry vehicles, and heat sinks and radiators.
Carbon composites are at present fabricated using either an impregnation method or the isothermal chemical vapor infiltration (CVI) process. G. Savage, Carbon-Composites, Chapman & Hall, NY, 1993. In the impregnation method, the carbon preforms are impregnated with resin or pitch followed by carbonization and graphitization. Both the resin and pitch shrink during the carbonization and graphitization steps, necessitating numerous cycles of impregnation and carbonization to obtain dense carbon composites. Also, the carbon matrix produced by pyrolysis of resin does not graphitize easily below 3000.degree. C. C. R. Thomas, ed., Essentials of Carbon-Carbon Composites, Royal Society of Chemistry, London, 1993. The isothermal chemical vapor infiltration process has been one of the most prevalent processes to fabricate carbon composites for high temperature applications. The main disadvantages of this process are long processing times (500-600 h), density gradients, and the need to machine the outer impermeable skin from the composite to facilitate infiltration.
Prior attempts have been made to overcome some of the above shortcomings with limited success. For example, a temperature gradient was applied across the thickness of the preform during the infiltration process so that a temperature at the surface away from the reagent source counteracts the effect of reduced reagent concentration due to diffusion, as shown by U.S. Pat. No. 5,348,774 to Golecki, et al. This results in more uniform densification. The processing time required by this temperature gradient process was found to be much shorter than isothermal CVI, however, the effect of diffusion becomes significant near the end of the infiltration causing significant reductions in the rate of densification. Kimura et al., High Temp. High Press., 13, 193 (1980) employed forced flow through the thickness of the preform and was able to reduce the processing time. But the depletion of reagent while flowing through the preform results in non-uniform densification and plugging of the flow path at the surface where the reagent first contacts the preform. Pulse CVD has also been used to infiltrate the composites. This process is described by R. L. Beaty and D. V. Kiplinger, Nucl. Appl. Tech., 8, 488 (1970); and K. Sugiyama and T. Nakamura, J. Mat. Sci. Lett. 6, 331, (1987). The main disadvantages of this method are the high number of pulses required to infiltrate the composites and the vacuum equipment. Hence, there is still a need for developing an infiltration process which will produce dense composites in a short time.
Carbon/carbon composites are at present fabricated using may cycles of resin or pitch impregnation followed by carbonization and frequently high temperature graphitization. Carbon/Carbon Materials and Composites, ed. by J. D. Buckley and Dan D. Edie, Noyes Publications, Park Ridge, N.J., pages 111-118 and 211-215 (1993). Since the resin and pitch shrink during the carbonization and graphitization steps, numerous cycles of impregnation have to be carried out to obtain sufficiently dense components. Isothermal chemical vapor infiltration (CVI) is also well established for the manufacture of carbon/carbon composites. In this process the reactors have to be operated at low pressures and temperatures in an attempt to obtain uniformly dense composites. The low temperatures coupled with low pressures lead to very low deposition rates, hence, this method requires infiltration times on the order of several weeks and is restricted to thin components. The limitations enumerated above for the various processes add considerably to the cost of the components and limit the application of this material. Some of these limitations can be overcome by using the forced flow-thermal gradient chemical vapor infiltration process which is often referred to as FCVI, as shown by U.S. Pat. No. 4,580,523 to Lackey et al. The main advantages of this process include reduced processing times due to forced flow of the reagent and uniform densification throughout the preform.
In the FCVI process a temperature gradient on the order of 200-500.degree. C. is applied across the preform and the reagent gasses are forced to flow through the preform from the cold to the hot surface, as shown by FIG. 9. Ideally, it is desirable to obtain uniform deposition throughout the preform. This can be accomplished by choosing the reagent concentration and the temperature as well as the flow rate such that the high reagent concentration at the cold side offsets the lower temperature, and the lower concentration at the hot side offsets the high temperature resulting in a uniform deposition rate. Also, the temperature gradient helps in prevention of the formation of an impermeable skin at the surface of the preform that is first exposed to the reagent. This methodology allows the process to be operated at much higher temperatures than the isothermal process, thereby further reducing the processing time significantly. The forced flow-thermal gradient technique also offers greater flexibility in the selection of the processing conditions. It is not essential to use the low temperatures, pressures, and reagent concentrations as in the isothermal process. Consequently, there is a wider latitude in selection of the conditions to obtain deposits possessing the required microstructure and properties. W. J. Lackey and T. L. Starr, in Fiber Reinforced Ceramic Composites, ed. by K. S. Mazdiyansi, 397-449 (Noyes Publications, NJ, 1991).
The forced flow-thermal gradient chemical vapor infiltration process (FCVI) which incorporates the advantages of both forced flow and thermal gradient processes, has been found to be very effective in rapidly fabricating uniformly dense SiC/SiC composites. See, for example, U.S. Pat. No. 4,580,523 to Lackey, et al. The FCVI process is also applicable to the fabrication of carbon/carbon composites using propylene, propane, or methane. S. Vaidyaraman, W. J. Lackey, G. B. Freeman, P. K. Agrawal and M. D. Langman, J. Mat. Res., in press (1995). The present invention provides an in-depth quantitative understanding of the FCVI processing variables, that is, temperature, propylene concentration, and total flow rate on the infiltration time, final porosity and uniformity of the densification.